Rubber vulcanizates having reduced heat evolution when subjected to dynamic deformation, a process for their preparation and their use

ABSTRACT

Rubber vulcanizates having a reduced heat evolution when subjected to dynamic deformation, prepared from vulcanizable rubber mixtures containing at least one type of rubber, a reinforcing resin system based on curable phenolic resins and other customary constituents, the vulcanizable rubber mixtures having been prepared from at least two rubber mixtures of different compositions. A process for their preparation by preparing in each case at least two different rubber mixtures differing from one another in their content of rubber, filler, oil, reinforcing resin system and, if appropriate, thermoplastic copolymer, by mixing the constituents thereof, then mixing the resulting mixtures with one another and vulcanizing the resulting final mixture. 
     The use of the vulcanizable rubber final mixtures for the production of mechanical rubber goods and tires.

Rubber vulcanizates having reduced heat evolution when subjected todynamic deformation, a process for their preparation and their use.

The invention relates to rubber vulcanizates composed of vulcanizablerubber mixtures containing at least one type of rubber, a reinforcingresin system based on curable phenolic resins and other customaryconstituents, the vulcanizable rubber mixtures having been prepared fromat least two rubber mixtures of different compositions, to a process fortheir preparation and to their use, in particular as components ofvehicle tires and as mechanical rubber goods.

The use of curable phenolic resins in the preparation of rubbervulcanizates has been known for a fairly long time. These products,which are usually also described as reinforcing resins, are phenolicresins of the novolak type and are cured, i.e. crosslinked, by means ofsuitable curing agents during the vulcanization of the rubber. Examplesof the curing agents employed are formaldehyde donors, such ashexamethylenetetramine and partly or completely etherified melamine andurea resins containing methylol groups. The use of such resin systems inthe rubber mixtures has the effect of improving certain importantmechanical properties of the vulcanizates, such as, for example, thestress moduli, and also increasing the hardness. On the other hand, whensubjected to repeated mechanical deformation, vulcanizates of this typeexhibit a high evolution of heat, which can result, in time, in animpairment of the overall properties or even in destruction of thematerial.

It has now been found, surprisingly, that the heat evolution of rubbervulcanizates of the same formulation when under dynamic loading isconsiderably lower if the rubber mixture employed has been prepared, notfrom a single mixture, but from at least two rubber mixtures ofdifferent compositions, of which, in the event that, for example, twodifferent mixtures are employed, the first mixture A contains 50 to 95parts by weight of rubber and 30 to 150 parts by weight of filler andthe other mixture B contains 50 to 5 parts by weight of rubber and 80 to5 parts by weight of filler, relative to 100 parts by weight of thetotal amount of rubber employed in the final mixture, and at least oneof these mixtures A or B contains 2 to 30 parts by weight of areinforcing resin system.

The behavior of the vulcanizates according to the invention isparticularly surprising, because the mechanical properties, such as, forexample, the hardness, the ultimate tensile strength, the elongation atbreak and the stress moduli do not differ essentially from those ofvulcanizates which, with the same formulation, have been prepared fromonly one single mixture instead of different mixtures of differentcompositions.

The invention therefore relates to rubber vulcanizates based on at leastone type of rubber and other customary cnstituents, the finalvulcanizable rubber mixture being prepared by mxing at least two rubberbase mixtures of different compositions in which the first base mixtureA contains 50 to 95 parts by weight of rubber and 30 to 150 parts byweight of filler, preferably carbon black, and a further base mixture Bcontains 50 to 5 parts by weight of rubber and 80 to 5 parts by weightof filler, preferably carbon black, relative to 100 parts by weight ofthe total amount of rubber employed in the final mixture, and at leastone of the base mixtures contains 2 to 30 parts by weight of areinforcing resin system.

The use of two different base mixtures for the preparation of thevulcanizable final mixture is particularly preferred.

In some cases, however, it can also be expedient and advantageous tostart from a larger number of different mixtures containing proportionswhich differ as widely as possible of rubber, active filler andreinforcing resin system, preferably from 3 to 5 different mixtureswhich can also differ in the type of rubber present in them, and tocombine these mixtures to form a vulcanizable final mixture according tothe invention.

The present invention makes it possible to prepare rubber vulcanizateswhich exhibit a lower heat evolution under dynamic loading and therebyhave a lower tendency to thermal degredation than comparablevulcanizates prepared from only one rubber mixture. Finished productsmade from the mixtures according to the invention thereby have a longerservice life and a higher use value. In addition, a lower proportion ofthe energy put into the vulcanizates when under dynamic loading isconverted into heat, so that there is also a lower consumption ofenergy, for example in the form of rolling resistance of vehicle tires,in the finished products. As part of the measures for saving energy invehicles, this constitutes an unexpected and surprising advantage.

The rubber mixtures according to the invention contain at least one typeof rubber and, as other constituents, customary substances, such asfillers, plasticizers, if appropriate thermoplastic copolymers,vulcanizing agents, accelerators, activators, reinforcing resin systemsand, if appropriate, processing aids or special additives, such as waxesand the like.

The invention also relates to a process for the preparation of therubber mixtures according to the invention from at least two differentbase mixtures, to their vulcanization and to their use, especially forthe preparation of vulcanizates for mechanical goods.

Suitable types of rubber are the types of rubber customarily used, inparticular, for example, natural rubber (NR), styrene/butadiene rubber(SBR), polybutadiene, polyisoprene, trans-polyoctenylene/butyl rubber,ethylene/propylene/diene terpolymer rubber or mixtures thereof, such asare used, for example, in the tire industry or for the preparation ofmechanical rubber goods. Styrene/butadiene rubber and natural rubber ormixtures of these types of rubber with minor amounts of other types ofrubber are preferred. They can be employed in any desired source form,for example in the form of balls or powder, and also, for example, witha content of carbon black.

In general, the reinforcing resins are phenol novolaks or other reactivepolyphenols or polynuclear phenols, such as resorcinol,diphenylolpropane or diphenylolmethane or naphthols and analogouscompounds. Examples of particularly suitable novolak resins are thosewhich can be obtained from phenol and/or polyhydric, mononuclear orpolynuclear phenols, such as, for example, resorcinol, bisphenols, suchas, for example, diphenylolpropane and/or diphenylolmethane, ormononhydric or polyhydric phenols which are substituted by a hydrocarbongroup, such as, for example, alkylphenols or aralkylphenols, preferablyhaving 1 to 20, especially 4 to 18 carbon atoms in the substituent, suchas, for example, tert.butylphenol, octylphenol, nonylphenol, cashew nutoil, styrenated phenol or phenol which has been modified with fattyacids, such as linseed oil fatty acids, or phenylphenol, by reactingthese with aldehydes, such as acetaldehyde, but preferably formaldehyde,in an acid medium. Phenolic resins modified with inorganic constituentsare also suitable for reinforcing rubber, preferably, for example, typesof phenolic resins modified with silicon, aluminum or boron compounds.The novolaks can also contain plasticizing components, such as, forexample, polyethylene glycols, tall oil or other customary plasticizers.Novolaks obtained from phenol and mixtures of phenol and phenolssubstituted by a hydrocarbon group having 4 to 18 carbon atoms in thesubstituent, including cashew nut oil, are preferred for practical use.The proportion of the substituted phenols in the novolak can vary, butthe novolak should still be capable of curing. In general, theproportion of nontrifunctional phenols in the total content of phenolsis not higher than 80 mol %, preferably not higher than 65 mol %.

In accordance with the invention, the novolaks are employed in at leastone of the base mixtures, preferably in an amount of 2 to 30, especially2.5 to 27 and particularly preferably 3 to 25, parts by weight, relativeto 100 parts by weight of the total amount of rubber employed in thefinal mixture.

The curing agents used for the phenol novolak can be any desiredcompounds which split off formaldehyde and/or free or etherifiedcompounds containing methylol groups, for example amine resins, such as,for example, urea resins or benzoguanamine resins.Hexamethylenetetramine (described in the following text as "hexa") andmelamine resins are preferred, however, the latter in the form ofnon-etherified or partly or largely etherified products and, ifappropriate, more highly condensed polynuclear products. The degree ofcondensation of the aldehyde resins, in particular formaldehyde resins,and their content of free methylol groups can vary within certainlimits. The free methylol groups can be etherified with monohydricalcohols having 1 to 12, preferably 1 to 8, carbon atoms, such as, forexample, methanol, ethanol, the various isomeric propanols, butanols,nonanols or dodecyl alcohol. Hexamethoxymethyl melamine, for example, iswidely used.

In general, the otpimum amounts of curing agent required for curing thenovolak resins must be determined in preliminary tests, which are easyto carry out. In general, about 2 to 60, preferably 5 to 50, % by weightof melamine resin or 2 to 30, preferably 2 to 25, % by weight ofhexamethylenetetramine are used, relative to the novolak resin.

In addition to active carbon blacks and silicas, the fillers used canalso be kaolins, chalk and/or other customary materials, as well aspigments. The use of carbon black is particularly preferred. In somecases the use of fillers is substantially dispensed with, for example intransparent mixtures. Suitable plasticizing oils are the productscustomary in the technology of rubber, especially naphthenic andaromatic oils.

The vulcanization of the rubber mixtures according to the invention canbe carried out without sulfur by means of vulcanization resins.Vulcanization in the presence of sulfur is preferred, however. Itsprogress can be adapted to suit the practical requirements by selectingsuitable accelerators, activators and regulators which are customary inthe technology of rubber. It can also be advantageous to add antiagingagents to the vulcanizable mixtures in order to improve the propertiesof the vulcanizates further.

The initial rubber mixtures employed in accordance with the invention,such as, for example, the base mixtures A and B, can each be prepared ina customary manner in internal mixers or on mixing rolls. When thereinforcing resins are incorporated, it can be advantageous if themixing temperature in some phase of preparing the mixture exceeds themelting point of the resins, but higher hardness values can be achieved,if appropriate, also by means of formulations in which, after the resinshave been added, the mixing temperature remains below the melting point.It is also important that the curing agents should be incorporated underconditions such that premature self-condensation or a premature reactionwith the novolak to be cured does not take place. This can be achieved,for example, by incorporating the curing agents only at the end of themixing process at temperatures which are not too high (in general at 80°to 120° C.). When preparation of the mixtures is complete, theseparately prepared batches, such as, for example, the base mixtures Aand B, are combined, mixed with one another and finally, if appropriatewhile being shaped, vulcanized at an elevated temperature in a customarymanner.

The vulcanizates obtained in accordance with the invention can be used,for example, as components of vehicle tires, such as treads, side walls,bracing plies or cushions, and also as mechanical rubber goods, such as,for example, rubber sleeves, lining and conveyor belts. In this regard,the mixtures according to the invention can also be used in rubberlayers which have to exhibit good adhesion to inlaid woven or braidedmaterials. In order to improve the adhesion it is also possible, inaddition, to use adhesion promoters, such as, for example, cobaltcompounds or other metal compounds and/or combinations of silica,resorcinol and curing agent.

The invention is illustrated in greater detail by means of the followingexamples.

The tests described in the following examples serve to illustrate thecomposition of the rubber mixtures according to the invention and theproperties of the vulcanizates prepared therefrom. In each of theexamples according to the invention at least two different mixingbatches, batch A and batch B, were prepared, differing markedly in theircontent of rubber, carbon black, oil and reinforcing resin. Theresulting mixing batches A and B were then mixed with one another togive a combined or final mixture according to the invention and werevulcanized.

In the comparison examples, the vulcanizable combined mixture wasprepared, for comparison, in a single mixing batch, the percentagecomposition of which corresponds in each case to that of the finalmixture of one of the examples according to the invention, and theresulting combined mixture was vulcanized under conditions which werealso comparable.

The components listed as other constituents (OC) of the mixtures in theexamples or comparison examples were not varied and were added in thefollowing proportions by weight, relative to 100 parts by weight ofrubber:

    ______________________________________                                              2       parts by weight of stearic acid                                       3       parts by weight of zinc oxide                                         1       part by weight of N--isopropyl-N--phenyl-p-                                   phenylenediamine*                                                     1       part by weight of 2,2,4-trimethyl-1,2-dihydro-                                quinoline*                                                            1.8     parts by weight of sulfur                                             1.5     parts by weight of benzothiazole-2-cyclohexyl-                                sulfenamide**                                                         0.2     part by weight of mercaptobenzothiazole                         Total 10.5    parts by weight                                                 ______________________________________                                         *as antiaging agent                                                           **as accelerator                                                         

The mixtures were prepared on a mixing roll. Vulcanization was carriedout for 30 minutes at 155° C.

In the mixing process, the amount of carbon black still, if appropriate,lacking, and then the oil and the further components in the sequencestearic acid, zinc oxide and antiaging agent were added to the rubber orto a master batch containing carbon black. In the case of the batchescontaining a reinforcing resin, the latter was mixed in subsequently attemperatures between 90° and 100° C., and homogenization was carried outbriefly at 110° to 120° C. The sulfur, the accelerator and the curingagent were mixed in, in the case of all the batches, at the end of themixing process at temperatures below 100° C. The reinforcing resin usedwas a commercially available modified phenol novolak having a meltingrange from 70° to 80° C., measured by the capillary process, and aviscosity at 20° C. of 600 to 1,000 mPa.s in a 50% strength by weightsolution in ethylene glycol monoethyl ether. The curing agent used washexamethylenetetramine (hexa). When the final mixtures had beenvulcanized, the following properties were determined on the resultingvulcanizates by the test methods customary in practice: ultimate tensilestrength, elongation at break and stress moduli (at 10% to 300%elongation) as specified in DIN 53,504; Shore A hardness as specified inDIN 53,505; rebound resilience as specified in DIN 53,512; temperaturerise after 5 and after 15 minutes exposure to stress in a flexuralrolling test apparatus in which a test cylinder of diameter 20 mm andlength 100 mm is subjected to dynamic deformation at an angle ofdeflection of 45° at one end at a speed of approx. 1,400 revolutions perminute; and plasticity of the unvulcanized mixtures in a Mooneyshear-disk viscometer as specified in DIN 53,524.

Test results

The determinations were carried out on mixtures whose compositions areapparent from Tables 1 and 2. The results of the tests on thevulcanizates are also listed in Tables 1 and 2. Table 1 containsExamples 1 and 2 and Comparison Example 1 together with thecorresponding mixture compositions and the test results on thevulcanizates.

                                      TABLE 1                                     __________________________________________________________________________                   Example         Comparison                                     Constituents of mixture                                                                      1       2       Example 1                                      __________________________________________________________________________    Batch A                                                                       SBR rubber     80  parts                                                                             70  parts                                                                 by wt.  by wt.                                             carbon black N 330                                                                           60  parts                                                                             55  parts                                                                 by wt.  by wt.                                             aromatic oil   15  parts                                                                             10  parts                                                                 by wt.  by wt.                                             phenol novolak 9   parts                                                                             9   parts                                                                 by wt.  by wt.                                             hexa           1   part                                                                              1   part                                                                  by wt.  by wt.                                             other constituents                                                                           8.4 parts                                                                             7.35                                                                              parts                                              (OC)               by wt.  by wt                                              Batch B                                                                       SBR rubber     20  parts                                                                             30  parts                                                                 by wt.  by wt.                                             carbon black N 330                                                                           5   parts                                                                             10  parts                                                                 by wt.  by wt.                                             aromatic oil       --  5   parts                                                                         by wt.                                             phenol novolak     --      --                                                 hexa               --      --                                                 other constituents                                                                           2.1 parts                                                                             3.15                                                                              parts                                              (OC)               by wt.  by wt                                              Final                                                                         Mixture                                                                       SBR rubber     100 parts                                                                             100 parts                                                                             100 parts                                                         by wt.  by wt.  by wt.                                     carbon black N 330                                                                           65  parts                                                                             65  parts                                                                             65  parts                                                         by wt.  by wt.  by wt.                                     aromatic oil   15  parts                                                                             15  parts                                                                             15  parts                                                         by wt.  by wt.  by wt.                                     phenol novolak 9   parts                                                                             9   parts                                                                             9   parts                                                         by wt.  by wt.  by wt.                                     hexa           1   part                                                                              1   part                                                                              1   part                                                          by wt.  by wt.  by wt.                                     other constituents                                                                           10.5                                                                              parts                                                                             10.5                                                                              parts                                                                             10.5                                                                              parts                                      (OC)               by wt.  by wt.  by wt.                                     Mooney plasticity measured                                                    at 120° C.                                                             Batch A        40      35      --                                             Batch B        33      21      --                                             Final mixture  37      29      35                                             Vulcanization time                                                                           30 min  30 min  30 min                                         at 155° C.                                                             Test results on the                                                           vulcanizate:                                                                  Ultimate tensile                                                                             17.0    17.9    17.3                                           strength (MPa)                                                                Elongation at  327     425     469                                            break (%)                                                                     Stress modulus,                                                                              1.7     1.2     1.5                                            10% elongation (MPa)                                                          Stress modulus,                                                                              3.0     2.2     2.4                                            50% elongation (MPa)                                                          Stress modulus,                                                                              4.7     3.5     3.5                                            100% elongation (MPa)                                                         Stress modulus,                                                                              10.3    8.1     7.2                                            200% elongation (MPa)                                                         Rebound resilience                                                                           35      32      35                                             at 23° C. (%)                                                          Shore A hardness                                                                             82      77      86                                             at 23° C. (°)                                                   Temperature rise                                                              in the flexural                                                               rolling test                                                                  apparatus after                                                               5 minutes (°C.)                                                                       43      36      63                                             15 minutes (°C.)                                                                      54      47      73                                             __________________________________________________________________________

The lower temperature rise measured on the vulcanizates when subjectedto dynamic deformation at an identical composition of the final mixturescan be seen clearly from Examples 1 and 2 according to the invention,compared with the vulcanizate of Comparison Example 1 prepared from onlya single mixing batch. Thus, in the flexural rolling test, thetemperatures determined for comparison on the vulcanizates from therubber mixtures according to the invention of Examples 1 and 2 wereabout 20° C. or more below the temperature measured on the vulcanizateof Comparison Example 1.

Table 2 contains Examples 3 and 4 and Comparison Example 2, togetherwith the corresponding mixture compositions and the test results on thevulcanizates.

                                      TABLE 2                                     __________________________________________________________________________                   Example         Comparison                                     Constituents of mixture                                                                      3       4       Example 2                                      __________________________________________________________________________    Batch A                                                                       SBR rubber     80  parts                                                                             70  parts                                                                 by wt.  by wt.                                             carbon black N 330                                                                           50  parts                                                                             45  parts                                                                 by wt.  by wt.                                             aromatic oil   5   parts                                                                             5   parts                                                                 by wt.  by wt.                                             phenol novolak 18  parts                                                                             18  parts                                                                 by wt.  by wt.                                             hexa           2   parts                                                                             2   parts                                                                 by wt.  by wt.                                             other constituents                                                                           8.4 parts                                                                             7.35                                                                              parts                                              (OC)               by wt.  by wt.                                             Batch B                                                                       SBR rubber     20  parts                                                                             30  parts                                                                 by wt.  by wt.                                             carbon black N 330                                                                           5   parts                                                                             10  parts                                                                 by wt.  by wt.                                             aromatic oil       --  5   parts                                                                         by wt.                                             phenol novolak     --      --                                                 hexa               --      --                                                 other constituents                                                                           2.1 parts                                                                             3.15                                                                              parts                                              (OC)               by wt.  by wt.                                             Final                                                                         mixture                                                                       SBR rubber     100 parts                                                                             100 parts                                                                             100 parts                                                         by wt.  by wt.  by wt.                                     carbon black N 330                                                                           55  parts                                                                             55  parts                                                                             55  parts                                                         by wt.  by wt.  by wt.                                     aromatic oil   5   parts                                                                             10  parts                                                                             10  parts                                                         by wt.  by wt.  by wt.                                     phenol novolak 18  parts                                                                             18  parts                                                                             18  parts                                                         by wt.  by wt.  by wt.                                     hexa           2   parts                                                                             2   parts                                                                             2   parts                                                         by wt.  by wt.  by wt.                                     other constituents                                                                           10.5                                                                              parts                                                                             10.5                                                                              parts                                                                             10.5                                                                              part                                       (OC)               by wt.  by wt.  by wt.                                     Mooney plasticity measured                                                    at 120° C.                                                             Batch A        44      33      --                                             Batch B        33      21      --                                             Final mixture  31      28      26                                             Vulcanization time                                                                           30 min  30 min  30 min                                         at 155° C.                                                             Test results on the                                                           vulcanizate:                                                                  Ultimate tensile                                                                             17.4    17.2    15.0                                           strength (MPa)                                                                Elongation at  338     361     417                                            break (%)                                                                     Stress modulus,                                                                              2.4     2.1     2.8                                            10% elongation (MPa)                                                          Stress modulus,                                                                              4.2     3.7     3.7                                            50% elongation (MPa)                                                          Stress modulus,                                                                              6.3     5.9     4.9                                            100% elongation (MPa)                                                         Stress modulus,                                                                              11.3    10.9    8.3                                            200% elongation (MPa)                                                         Rebound resilience                                                                           38      37      38                                             at 23° C. (%)                                                          Shore A hardness                                                                             90      87      88                                             at 23° C. (°)                                                   Temperature rise                                                              in the flexural                                                               rolling test                                                                  apparatus after                                                               5 minutes (°C.)                                                                       71      62      91                                             15 minutes (°C.)                                                                      89      76      105                                            __________________________________________________________________________

Examples 3 and 4 were carried out using a higher added amount ofreinforcing resin, with the result that the vulcanizates 35 had bothhigher stress moduli and higher hardness values. Here too the heatevolved in the flexural rolling test in the case of the vulcanizatesprepared in Examples 3 and 4 according to the invention was markedlylower when compared with the vulcanizate of Comparison Example 2.

I claim:
 1. A process for the preparation of a rubber vulcanizate fromvulcanizable rubber mixtures containing at least one rubber andconventional compounding ingredients, which comprises preparing in eachcase at least two different rubber mixtures A and B of differentcompositions, the first mixture A containing 50 to 95 parts by weight ofrubber and 30 to 150 parts by weight of filler, and a further mixture Bcontaining 50 to 5 parts by weight of rubber and 80 to 5 parts by weightof filler, relative to 100 parts by weight of the total amount of rubberemployed in the final mixture, and at least one of the base mixturescontaining 2 to 30 parts by weight of a reinforcing novolak resin systemand differing from one another in their content of at least one, of thecomponents rubber, filler, oil, reinforcing resin system optionallythermoplastic copolymer, by mixing the constituents thereof, then mixingthe resulting mixtures with one another and vulcanizing the resultingfinal mixture at an elevated temperature.
 2. The process of claim 1,wherein the reinforcing resin system is composed of a novolak and acuring agent which is incorporated together with this novolak orseparately.
 3. The process of claim 1, wherein the curing agent iscomposed of hexamethylenetetramine or a melamine resin.
 4. The processof claim 1, wherein the novolak in mixture A is employed in an amount of0 to 50 parts by weight, relative to 100 parts by weight of the totalamount of rubber employed in the final mixture.
 5. The process of claim1, wherein the novolak in mixture B is employed in an amount of 10 to 60parts by weight, relative to 100 parts by weight of the total amount ofrubber employed in the final mixture.
 6. The process of claim 1 whereinthe novolak is composed (a) of phenol or mixtures thereof with phenolswhich are substituted by a hydrocarbon group having up to 18 carbonatoms, and (b) formaldehyde.
 7. The process of claim 1 wherein therubber is composed of styrene/butadiene rubber and/or natural rubber ormixtures of said rubbers together with minor amounts of other rubbers.8. The process of claim 1, wherein the active inorganic filler iscomposed, at least predominantly, of carbon black.